The use of carbon discs as a friction element in aircraft wheel and brake assemblies have gained acceptance as a way of increasing the payload of an aircraft by reducing the weight of braking systems.
In U.S. Pat. No. 3,639,197, it is disclosed how a continuous carbon fiber can provide a carbon disc with the structural unity needed to absorb repeated braking torque. Such carbon friction disc when used in aircraft braking systems often experience thermal conditions above 800.degree. F. Above 800.degree. F. the non-swept areas of carbon friction disc are subject to accelerated oxidation which can lead to a disintegration of the carbon matrix and continuous carbon fiber resulting in a brake element failure.
In U.S. Pat. No. 3,914,508 a method is disclosed for protecting a carbon substrate in a moist environment from oxidation by coating a selected surface thereof with a boron and metallic mixture attached thereto by a resin matrix. As with most coatings, it is difficult to maintain a uniform thickness over the entire peripheral surface. As a result where the coating is limited or absent, deterioration of the carbon substrate often takes place after repeated applications.
In a further attempt to protect a carbon friction material, a metal ring as disclosed in U.S. Pat. No. 3,473,637, was placed on the outer periphery of a carbon disc to prevent oxidation of the non-swept area. During frictional operation when the thermal energy produced is low, such protection is effective. However, as the thermal energy generated during braking increases, the unity of the carbon material and steel ring change in a direct proportion to the differences in their thermal coefficients of expansion. After a repeated number of frictional engagements at high temperatures, structural defects can occur along the periphery of the carbon material because of different coefficients of expansion. Thereafter, oxygen in the surrounding air can enter and degrade the underlying carbon friction disc.
U.S. Pat. No. 3,972,395 discloses a protection member which matches the coefficient of thermal expansion of the carbon friction disc. The protection member, which includes a woven carbon sheath and a protecting screen, is bonded to the peripheral non-swept surface to protect the underlying carbon friction disc. In order to protect the driving slots on the rotor member, a reinforcing plate is attached to the peripheral surface by a driving pin that extends through the carbon friction disc. Unfortunately, the driving pins are located in a high stress area and under some extreme conditions a structural failure may occur in the area of the driving pins.